Lunch poaching is an age old sport in some offices. It's a common problem, though I was lucky enough to not suffer from it; I like to think it's because my office was an honest lot, but perhaps my lunches were just not all that appetizing. ;)
However, there is a solution that is both ingenious and revolting at the same time! (And who doesn't want that?!?) Enter the moldy sandwich bag. It's a perfectly clean sammich bag that has been doctored to look like its contents are inedible--or perhaps contain the cure to pneumonia. I'm not kidding. Here, see:
I guess it'd be effective. Unless you forget about it and leave it overnight or have an overzealous fridge cleaner in your midst. Then your lunch is trash!
Personally, I am not sure I could get over the appearance of this. I would just keep an insulated bag at my own desk if it were that big an issue.